The use of electrostatic latent images to cause liquid developer containing electrically charged, pigmented toner particles to form a visible image is well known. It is well known in the art to accomplish such printing by first creating a latent image on an intermediary surface, toning the latent image to make a visible image on the surface and then transferring that visible image to a recording medium such as paper. It is also known to form an image directly on a recording medium by depositing an electrostatic image onto a surface of that medium and then applying such toner to that surface. The former system is sometimes termed "transfer process" printing, whereas the latter system may be termed "direct process" printing.
The liquid developer is typically composed primarily of a non-polar, dielectric carrier liquid, such as "Isopar", a trademark of Exxon Corporation for certain aliphatic hydrocarbon liquids in which pigmented toner particles and charge agents are dispersed to make developer. The toner particles are typically combinations of polymer resins and colored pigments which adsorb the charge agents, thereby causing the particles to acquire an electrical charge and enabling their response to an electrostatic image. Typically, a large number of charge agent molecules adsorb onto a toner particle causing a small degree of ionization. Literally millions of charge agent molecules may attach themselves to a colored particle but the number of ionic charges produced is typically only about 10 to 100. The great majority of adsorbed charge agent molecules remain electrically neutral. Also, for each ionic charge produced on the surface of the particle a corresponding counter ion of opposite charge is produced in the surrounding carrier liquid. In this manner the developer as a whole remains electrically neutral. Over time thermodynamic equilibrium is reached and essentially every toner particle becomes charged. Some of the charge agent molecules may remain in solution rather than adsorbed onto a particulate surface.
Also incorporated in some liquid developers are stearic stabilizers, which are not electrically active but which coat the toner particles so as to prevent them from agglomerating in a hard mass, so that a small amount of stirring of the developer causes redispersion. The stearic stabilizers also serve to fix the dried image by gluing the toner particles to the surface of the recording paper. However, the stearic stabilizers may interfere with the adsorption and desorption of charge agents.
The developer is often supplied to a printing apparatus as a "pre-mix" because of a delay of perhaps one week after mixing before it is stabilized and thermodynamic equilibrium is reached. In this manner it can be used immediately without waiting for stabilization of the developer. This developer is held in a tank in the apparatus, and then pumped to an applicator for printing on the recording medium. Some printing apparatuses have a separate tank containing liquid carrier, and another tank containing a developer concentrate. For a color printer, a set of developers are contained in a set of tanks, each tank housing a developer of a different primary color, and a separate concentrate tank may be provided corresponding to each developer tank, with a wash tank for carrier liquid also provided.
The concentrate is essentially comprised of the elements contained in the developer but with a greatly reduced amount of carrier. The concentrate also typically contains a higher concentration of charge agent than does the developer. This excess charge agent is provided to keep the toner particles in an homogenous suspension so as to prevent their settling out. This higher level of charge agent causes an increased level of charge on the surfaces of the toner particles in the concentrate mix. The higher charge level causes increased mutual repulsion of the particles from each other so as to stabilize the concentrate liquid and delay or prevent particle settling and agglomeration into a solid mass or cake. The relatively high level of charge agent in the concentrate is also necessary because the concentrate, unlike the developer, is not stirred by repeated pumping to and from the applicator. Typically, the concentrate rests in a quiescent state and is only slightly disturbed when a small amount of concentrate is injected into a stream of developer as demanded by usage of a particular color. This quiescent state provides opportunity for particulate settling. If such settling were to occur it would be difficult or impossible for the injection system to work successfully.
After passing through the toner applicator to form an image, the spent liquid developer minus whatever toner particles were used in visible image formation is returned to its developer tank to be reused. In some printing systems, a small amount of concentrate is added to replenish the toner particles transferred to the medium, and some carrier is similarly added to the developer, the relative amounts added commonly dictated by optical sensing of toner particle concentration in the developer and by overall liquid level sensing in the developer tank.
As the developer is gradually replenished with concentrate, the higher concentration of charge agent in the concentrate raises the concentration of charge agent in the developer. The typically higher level of charge agent in the concentrate mix leads to various problems.
As the developer is used there is a steady increase in the concentration of charge agent in the developer tank. The excess charge agents in solution may become partially ionized leading to clear phase electrical conductivity which tends to compete directly with charged toner particles in responding to and neutralizing the electrostatic image. Hence, as the concentration of charge agent in the developer increases, the image produced on the recording medium becomes fainter because latent image neutralization is accomplished by both colored particles and by colorless ions. As more colorless ions replace more and more colored particles in the visible image, that image is seen to fade away.
The higher concentration of charge agent in the developer also leads to excess charge of the colored particles themselves. Even if there were no increase in colorless ions or in clear phase electrical conductivity, there would be fewer colored particles required to neutralize the latent image and the image would be seen to fade away. The increasing level of charge agents in the developer would lead to an increasing charge-to-mass ratio for the toner particles so that fewer of them would be able to neutralize the latent image. Thus, both an increase in colorless ions and increased particulate charge independently lead to image fading.
Furthermore, if a great amount of concentrate is added quickly to the developer as in the case of heavy usage of one color, the added colored particles do not have time to thermodynamically equilibrate to the reduced charge level of the developer, that is, they temporarily carry excess charge even while their charge level is continuously dropping to the lower (but increasing) level of the developer. This transient excess charge level of the newly added particles contributes still further to image fading.
In direct process printing the liquid developer also picks up ionic contaminants from the recording medium directly. These contaminants also build up with repeated printing applications, additionally fading the prints because they, too, compete with the charged toner particles to respond to the oppositely charged electrostatic image.
Thus there are several factors which contribute to image fading as more and more prints are made. The increased concentration of competing ions and continuously increasing charge level of the colored particles causes image fading which cannot be corrected by adding of concentrate. The only practical solution is to replace and discard the ruined developer. This is expensive and environmentally undesirable.
In U.S. Pat. No. 4,980,259, Landa et al. teach a formula for an amount of charge director, another name for charge agent, to be supplied to a batch of concentrate during manufacture to attempt to maintain a constant charge director level in the developer. This concentrate is then added to a developer liquid, along with a mixture of carrier liquid and charge director, in response a measurement of the overall amount of the developer and a measurement of the concentration of toner particles in the developer. U.S. Pat. No. 5,155,001, also to Landa et al., teaches supplying an excess of a partially soluble charge director compound to a carrier liquid, so that the charge director remains at a saturation concentration in the carrier liquid.
A remaining problem is how the charge director concentration in the developer can be at a low enough level to prevent fading of the printed image while the charge director concentration in the concentrate is high enough to prevent settling and agglomeration of the toner particles.
It is an object of the present invention to provide a printing system that can be operated continuously without discarding developers in bulk due to excess charge agent. It is a further object of this invention to provide a direct process printing system that can be operated continuously without discarding developers in bulk due to an increase of ionic contaminants.